1. Field of the Invention
This invention relates to nonmagnetic ceramics and ceramic multilayer parts having inductor sections formed using the nonmagnetic ceramics.
2. Background Art
Ceramic multilayer parts such as chip filters and chip inductors are widely employed with the advance of the technology of fabricating electronic parts as surface mount devices (SMD) and to meet the demand for higher performance on high-frequency parts as used in communication equipment. Inductor sections of chip filters and chip inductors are manufactured, without using windings, by depositing alternating layers of magnetic paste and conductive paste and firing the layers for integration.
For the ceramic magnetic layers in the inductor sections of ceramic multilayer parts, Ni--Cu--Zn ferrites and analogous materials are generally selected because they can be fired at low temperatures and have relatively superior high-frequency characteristics. Also, silver or silver alloy having a low resistivity is used as the conductive material for the internal conductor.
However, since Ni--Cu--Zn ferrites have a dielectric constant E of about 10 to about 15, it is difficult to reduce the floating capacitance between internal conductor patterns when the spacing between the conductor patterns is narrow. For this reason, in the event of narrow internal conductor patterns, the self-resonant frequency cannot be high and the use at high frequency is restricted.
Under such circumstances, JP-A 7809/1992 proposes "a multilayer ceramic inductor comprising a conductor pattern turned between insulator layers, with edges interconnected, in an overlapping manner in a stacking direction, characterized in that the material of which said insulator layers are formed is a nonmagnetic ceramic material." It is described that exemplary of the nonmagnetic ceramic material used therein are "mixtures of glass and cordierite" and "mixtures of glass and cordierite with mullite added," and as the glass, borosilicate glass containing 63 to 85% by weight of SiO.sub.2 and 3 to 28% by weight of B.sub.2 O.sub.3 is preferred. Allegedly, the use of such nonmagnetic ceramic materials in the insulator layers allows the insulator layers to reduce their dielectric constant and increase their self-resonant frequency so as to comply with the high-frequency band, and also allows for low-temperature firing which in turn, enables the use of silver electrodes.
However, where the nonmagnetic ceramic materials described in the above-referred publication are used, multilayer ceramic inductors cannot have a fully high flexural strength. As a result, the deflective strength required for surface mount parts would become insufficient. The dielectric constant is about one-half of that of ferrite-containing materials, that is, .epsilon.=about 5.5 to about 6.5, which values are not sufficient for use in parts incorporated in high-frequency circuits as in cellular phones.
Also, using a composition of borosilicate glass and fused SiO.sub.2, the inventor previously accomplished a dielectric constant .epsilon. of 4.2. This composition, however, had the following problems. In compositions comprising borosilicate glass and SiO.sub.2 glass serving as nonmagnetic ceramic materials, when SiO.sub.2 glass is added in an amount of 25% by weight or more, the growth of cristobalite crystals occurs upon sintering. At this point, crystals having a high crystallinity localize or concentrate about silver serving as the conductor. The change of coefficient of thermal expansion caused by the cristobalite crystal growth becomes outstanding when the temperature rises from 100.degree. C. to 300.degree. C., and the coefficient increases from 0% to 0.260% or 0.270%. As a result, the rapid expansion by the cristobalite crystal growth localized about the conductor generates internal stresses, and even causes chip deformation or crack generation near the conductor particularly when inductors having an increased number of turns are fabricated.